Single-mode large mode area (LMA) fibers, which are important for high power fiber lasers, can maintain large mode areas when straight, but are poor at maintaining large mode areas when bent. Likewise, very large mode area (VLMA) fibers that have been fabricated so far have extremely high mode losses and low values for mode areas when bent. Successful design and fabrication of a bendable, large mode area fiber will not only improve compact, high-power fiber lasers, but also improve other applications, which rely on bendable optical fibers.
In addition, in a single-mode fiber amplifier or laser, limitations in scaling of the output powers come from the high intensities in the fiber, which can cause thermal and optical damage and also limit power scaling due to the onset of various nonlinear mechanisms. Nonlinear processes such as stimulated Raman scattering (SRS), self-phase modulation (SPM), and—particularly for narrow-linewidth/single-frequency laser operation—stimulated Brillouin scattering (SBS) are limiting factors. The impact of these nonlinear effects is proportional to the optical intensity. Thus, the effective mode area, Aeff, needs to be scaled for the attainment of high output powers from such fiber lasers to minimize the intrafiber intensity and thus the impact of various nonlinear optical effects. Consequently, a requirement for a multi-kW single element fiber laser/amplifier is that the mode field diameter be >15 μm, although much larger mode field diameters (>30 μm are preferable).
The use of practical doping levels and of double-clad and triple-clad fiber geometries, in which the outer claddings facilitate high-efficiency coupling of high power pump diodes into the fiber—and the requirement of near-complete absorption of the pump diode radiation to maximize efficiency—necessitates the use of fiber lengths that are usually of the order of a few meters. In order to achieve practical and compact fiber laser designs that are both rugged and portable, it is highly desirable that VLMA fibers are bendable to coil radii of 30 cm or smaller, with smaller fiber coil diameters (typically <40 cm) being preferable.